The invention relates to means which enables a signal to be given whether electric contacts between two parts in relative rotary movement are open or closed; it is of particular use in an automobile to indicate that a limiting value has been reached by a parameter related to the behaviour of its tires in such a way as to activate a sound and/or light wavering device provided for this purpose in the interior of the said vehicle.
Devices of this type have already been proposed, particularly for detecting an abnormal drop in the pressure in a tire. In such devices a pressure contact connected to the valve of the tire being monitored is in series in the circuit of a coil which is movable with the wheel of the vehicle and thus passes at regular intervals in front of two devices mounted facing the said coil on the chassis of the vehicle, the first device inducing in the mobile coil a current which is then coupled to the second or receiving device. The position of the pressure contact changes when the pressure inside the tire reaches a predetermined minimum value, thereby bringing about the presence or absence of the current induced by the mobile coil in the receiving device. The receiving device then processes the information thus provided and translate it so that it can be used by alarm means.
Different solutions have been proposed for the receiving device. One of these solutions is the use of an oscillatory circuit whose oscillations depend upon the current induced by the mobile coil. The inconvenient aspects of such an arrangement result from the severe environment in which it is implanted. The mobile coil which should be linked to the movement of the wheel is in fact fixed either on the rim or on the moving part of the braking system; this means that the fixed devices can only be implanted on or near the fixed parts of the brakes. It will be realised that situated here the electronic circuits must be subjected to exceedingly high temperatures (which can exceed 500.degree. C) which causes them to deteriorate rapidly. Another solution therefore also uses coils for said first and second devices so as to bring only the transmitting and receiving coils close to the mobile coil; the electronic circuits to which they are connected can be in a less exposed part of the vehicle. A known device of this type comprises a mobile coil wound on the rotating part of the braking system and working in co-operation with two fixed coils, one of which is a transmitter, the other a receiver, mounted on a part of the vehicle chassis and diametrically opposed with regard to the circumference defined by the mobile coil; the diameter being great enough to prevent all disturbing interaction of the transmitting coil on the receiving coil. It will be realised that it is awkward to mount such a device, and is in fact only practisable if done originally on a new vehicle. Any necessary repairs to the coils also present difficulties.